The invention relates to an electro-optical modulator based on a liquid crystal cell, which can be actively switched on and off with the aid of electric pulses.
In conventional electro-optical modulators based on liquid crystal cells, electric fields applied to nematic or nematic-cholesteric materials significantly influence their optical properties, such as light scattering, reflectance, birefringence or color. In particular, such modulators have become very important wherein the liquid crystal materials employed have a positive dielectric anisotropy (positive DKA), and the molecules of such liquid crystals, in the field-free state, are oriented with their longitudinal axes parallel to the electrode surface. Whenever the direction of that orientation is different at the opposite electrode surfaces, the axes of the liquid crystal molecules twist from one electrode surface to the other through a certain angle, preferably 90.degree.. Such twisted nematic cells are widely used for display elements of digital clocks or electronic computers.
The versatility of modulators of this type would be increased if it were possible to adjust the orientation of the liquid crystal molecules at will with respect to the electrode surfaces between the limiting values of being parallel or perpendicular to such surfaces. This would allow to vary the threshold voltage in the case of a given liquid crystall material to thereby reduce switching times.
For example, the number of the interference colors which appear when using the Freedericksz effect or the effect of the deformation of aligned phases (DAP effect), can be influenced by a freely selectable orientation of the molecules. These effects play an important part in the design of colored liquid crystal displays. If more than two orders of interference colors appear, pure color shades can be achieved only with difficulty in the case of liquid crystal display elements which operate on this basis. Therefore, mixtures of liquid crystal materials with dichroitic dyestuffs have hitherto been used predominantly for so-called color switches, although the liquid-crystalline properties of the base materials are as a rule adversely affected by added dyestuffs.
Were it possible to vary the threshold voltage, the operating voltage across a liquid crystal cell could be varied. Furthermore, if it were possible to influence the threshold voltage at discrete points of a display element in different ways, the display contrast in a liquid crystal cell could be improved markedly.
Finally, if it were possible to shorten the switching times, the number of switching steps per unit time could be increased thereby increasing the number of information items which could be transmitted through a display element.
The switching time of a liquid crystal cell is composed of the switch-on time and the switch-off time. The term "switch-on time" as used herein means that period of time between the instant of switching on the electric field and the instant at which 90% of the maximum display contrast is reached. Analogously, the switch-off time is the period of time between the instant of switching off the electric field and the instant at which the contrast ratio has decaved by 90%. Of these two components of the switching time, only the switch-on time is influenced by the level of the voltage applied. In conventional liquid crystal display elements, quite acceptable switch-on times of the order of magnitude of 1-10 milliseconds (ms) are achieved. By contrast, the switch-off time lies as a rule between 30 and 300 ms and essentially depends on the thickness of the liquid crystal layer and on the viscosity and the magnitude of the constant of elasticity of the liquid crystal material used, but it does not depend on the level of the control voltage applied.
It has already been proposed in German Auslegeschrift No. 2,450,390 to overcome this disadvantage in the design of a light barrier based on a liquid crystal cell by providing two liquid crystal cells, which can be separately triggered, one behind the other in the light beam. Both of the liquid crystal cells contain liquid crystal layers which are each twisted in themselves by 90.degree. and are arranged, in conjunction with polarizers, in such a way that no light can pass through in the field-free state. As soon as one of these cells is activated by an electric field, the arrangement becomes transparent after the switch-on time has elapsed. By a subsequent activation of the second cell, after a second switch-on time has elapsed, the arrangement again becomes impervious to light. It is evident that such an arrangement of two liquid crystal cells is very expensive to construct and therefore has a severely limited applicability.
It is the object of the invention to provide a liquid crystal cell based electro-optical modulator which provides control over the initial orientation of the liquid crystal molecules relative to the electrode surfaces, the threshold voltage or the switch-off time, or simultaneously even two of these properties.
Another object is the provision of a novel method for regulating the switch-off times of liquid crystal cells and for improving the contrast of matrix-addressed liquid crystal cells. Other objects will be apparent to those skilled in the art to which ths invention pertains.
These objects are achieved according to this invention, by a liquid crystal cell which contains a thin layer of a liquid crystal material with a positive DKA between transparent electrode base plates, each of which have one layer of an electrically conductive transparent electrode material to which an electric voltage can be applied, and in which the electrode plates are enclosed like a sandwich by polarizers, two magnets being provided, parallel or perpendicular to the surfaces of the electrode base plates, and the liquid crystal material having a negative anisotropy of the diamagnetic susceptibility (negative DMA).